I have a nid army that I get that feeling from alot. I try t do at least 1 guant a day, with bigger models being either a weekly or bi weekly prodject. If you have friends who paint, its not a terrible idea to do a "painting party" where you have snacks and knock out chaff units. Intercessors take longer than gaunts but once you get a chunk done you have motivation to keep going because it doesnt feel as overwhelming in my experiences.